In a known slip-controlled brake system of this type (DE-OS [German Published Patent Application] No. 3 627 000), a tandem master cylinder with a pneumatic brake booster connected upstream is used as a braking pressure generator. The auxiliary pressure supply system contains two hydraulic pumps allocated respectively to one associated brake circuit connecting the wheel brakes diagonally. In case of a slip-controlled brake operation, an auxiliary pressure, proportional to the foot pressure, is introduced by means of control valves from this auxiliary pressure supply system. This dynamic auxiliary pressure, on the one hand, is transferred to the static brake circuits connected to the master cylinder. On the other hand, this dynamic pressure acts on the wheel brakes. Inlet valves are inserted into the hydraulic circuit for the purpose of slip control. Normally, the inlet valves are open and it is possible to block pressure medium delivery to the respective wheel brake by means of the inlet valves upon an imminent lock-up of a wheel.
Further, outlet valves are provided at the wheel brakes. If necessary, it is possible to drain pressure medium from the wheel brake to the storage reservoir via the outlet valves. Upon the onset of slip control, the auxiliary pressure pumps are activated via an electric motor, and a controlled pressure is introduced into the wheel brakes via the inlet and outlet valves. For reasons of safety, moreover, the pistons within the tandem master cylinder are reset or arrested by means of the dynamic hydraulic circuit.
A disadvantage of the brake system described above is that, in dependence on the dynamic flow conditions and on the pressure modulated by the operating frequencies of the valves, sound vibrations are transmitted to the brake system via the fluid so that the parts associated with the brake system which constitute an oscillatory circuit, tend to build up resonance vibrations, allowing for their specific natural number of oscillations. Apart from the mechanical strain to be considered in this context and caused by pressure pulses of varying intensity, the resulting noise level is considered to be inconvenient and should be reduced. The relatively high delivery pressure of the auxiliary pressure pump constitutes a further disadvantage. The pressure developed is not adapted to the actual needs during brake slip control and can thus contribute to the increase in the noise level.